|String Theory is poorly written
You can help the Quantum Leap Wiki by rewording it.
The String Theory was the theory invented by Sam Beckett that inspired him to create Project Quantum Leap. He demonstrated his theory with a string. One end of the string represented birth, the other end represented death. Tie the ends together, and what you get is a loop. Ball the loop, and all the days of your life touch each other. Therefore, one could leap from one day to another in his/her own lifetime.
Apparently, young Sam Beckett got the idea for his String Theory from watching the 1950s television series Captain Galaxy. After he sent in a letter to the star of the show Captain Galaxy (Moe Stein) asking about Time Travel. However, older Sam was also the one who explained this time travel theory to Captain Galaxy/Moe Stein when he leaped into the body of Future Boy (Kenny Sharp). This is a Time Paradox.